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Why join?

"This will change the life of this nation for good. I welcome you wholeheartedly," said EU council president Herman Van Rompuy.

"Welcome to the European Union!" said EU commission president Jose Manuel Barroso.

Croatia is the first country to join the EU since 2007, when Bulgaria and Romania came on board, and only the second of the seven ex-Yugoslav states after Slovenia was part of the big bang eastward enlargement in 2004.

The path to join has been more exacting, with the EU having learnt that once countries join the bloc they lose momentum to reform and, particularly, to tackle corruption.

Zagreb formally applied for EU membership in 2003. Much of process concerned how it dealt with accused war criminals. Only two years later did the EU open accession talks with Croatia after the country was deemed to be fully cooperating with the UN war crimes tribunal in The Hague.

The reams of EU laws and reforms the country has to take on board have not always been popular. One particularly bitter pill to swallow was an EU demand for Croatia to close its debt-ridden, but popular, shipyards.

For their part, Croatian politicians have emphasized the importance of all countries in the region joining the bloc.

"We don't want Europe to stop at our borders, it must be open to other countries," President Ivo Josipovic said Sunday.

The country's membership comes at a difficult time for both sides.

The EU is mired in high unemployment and low growth, and in an almost constant state of bickering as it tries to overcome its Eurozone crisis and integrate further.

Croatia, with a population of 4.4 million, has an unemployment rate of around 20 percent. Its economy has for the past four years either been in recession or stagnant.

Josipovic pledged not to "let the cloud of the economic crisis overshadow our vision and optimism."

"The crisis is a challenge, an invitation to make tomorrow better than today," he said, according to AFP.